Radiation climate map for analyzing risks to astronauts on the mars surface from galactic cosmic rays

The potential risks for late effects including cancer, cataracts, and neurological disorders due to exposures to the galactic cosmic rays (GCR) is a large concern for the human exploration of Mars. Physical models are needed to project the radiation exposures to be received by astronauts in transit to Mars and on the Mars surface, including the understanding of the modification of the GCR by the Martian atmosphere and identifying shielding optimization approaches. The Mars Global Surveyor (MGS) mission has been collecting Martian surface topographical data with the Mars Orbiter Laser Altimeter (MOLA). Here we present calculations of radiation climate maps of the surface of Mars using the MOLA data, the radiation transport model HZETRN (high charge and high energy transport), and the quantum multiple scattering fragmentation model, QMSFRG. Organ doses and the average number of particle hits per cell nucleus from GCR components (protons, heavy ions, and neutrons) are evaluated as a function of the altitude on the Martian surface. Approaches to improve the accuracy of the radiation climate map, presented here using data from the 2001 Mars Odyssey mission, are discussed.     

Large solar flare radiation shielding requirements for manned interplanetary missions

As the 21st century approaches, there is an ever-increasing interest in launching manned missions to Mars. A major concern to mission planners is exposure of the flight crews to highly penetrating and damaging space radiations. Beyond the protective covering of the Earth's magnetosphere, the two main sources of these radiations are galactic cosmic rays and solar particle events. Preliminary analyses of potential exposures from galactic cosmic rays (GCR's) were presented elsewhere. In this Note, estimates of shielding thicknesses required to protect astronauts on interplanetary missions from the effects of large solar flare events are presented. The calculations use integral proton fluences for the February 1956, November 1960, and August 1972 solar particle events as inputs into the NASA Langley Research Center nucleon transport code BRYNTRN. This deterministic computer code transports primary protons and secondary protons and neutrons through any number of layers of target material of arbitrary thickness and composition. Contributions from target nucleus breakup (fragmentation) and recoil are also included. The results for each flare are presented as estimates of dose equivalent [in units of roentgen equivalent man (rem)] to the skin, eye, and bloodforming organs (BFO) behind various thicknesses of aluminum shielding. These results indicate that the February 1956 event was the most penetrating; however, the August 1972 event, the largest ever recorded, could have been mission- or life-threatening for thinly shielded (< or = 5 g/cm2) spacecraft. Also presented are estimates of the thicknesses of water shielding required to reduce the BFO dose equivalent to currently recommended astronaut exposure limits. These latter results suggest that organic polymers, similar to water, appear to be a much more desirable shielding material than aluminum.     

Radiation exposure predictions for short-duration stay Mars missions

The human radiation environment for several short-duration stay manned Mars missions is predicted using the Mission Radiation Calculation (MIRACAL) program, which was developed at NASA Langley Research Center. This program provides dose estimates for galactic cosmic rays (GCR) and large and ordinary solar proton flare events for various amounts of effective spacecraft shielding (both operational and storm shelter thicknesses) and a given time history of the spacecraft's heliocentric position. The results of this study show that most of the missions can survive the most recent large flares (if they were to occur at the missions' perihelion) if a 25 g/cm2 storm shelter is assumed. The dose predictions show that missions during solar minima (when solar flare activity is the lowest) are not necessarily the minimum dose cases, due to increased GCR contribution during this time period. The direct transfer mission studied has slightly lower doses than the outbound Venus swingby mission [on the order of 10-20 centi-Sieverts (cSv) lower], with the greatest dose differences for the assumed worst case scenario (when the large flares occur at perihelion). The GCR dose for a mission can be reduced by having the crew spend some fraction of its day nominally in the storm shelter (other than during flare events).     

The Radiation Assessment Detector (RAD) Investigation

The Radiation Assessment Detector (RAD) on the Mars Science Laboratory (MSL) is an energetic particle detector designed to measure a broad spectrum of energetic particle radiation. It will make the first-ever direct radiation measurements on the surface of Mars, detecting galactic cosmic rays, solar energetic particles, secondary neutrons, and other secondary particles created both in the atmosphere and in the Martian regolith. The radiation environment on Mars, both past and present, may have implications for habitability and the ability to sustain life. Radiation exposure is also a major concern for future human missions. The RAD instrument combines charged- and neutral-particle detection capability over a wide dynamic range in a compact, low-mass, low-power instrument. These capabilities are required in order to measure all the important components of the radiation environment. RAD consists of the RAD Sensor Head (RSH) and the RAD Electronics Box (REB) integrated together in a small, compact volume. The RSH contains a solid-state detector telescope with three silicon PIN diodes for charged particle detection, a thallium doped Cesium Iodide scintillator, plastic scintillators for neutron detection and anti-coincidence shielding, and the front-end electronics. The REB contains three circuit boards, one with a novel mixed-signal ASIC for processing analog signals and an associated control FPGA, another with a second FPGA to communicate with the rover and perform onboard analysis of science data, and a third board with power supplies and power cycling or “sleep”-control electronics. The latter enables autonomous operation, independent of commands from the rover. RAD is a highly capable and highly configurable instrument that paves the way for future compact energetic particle detectors in space.     

2001 Mars Odyssey Mission Summary

The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months.     

Martian Radiation EnvIronment Experiment (MARIE)

Measurements of radiation levels at Mars including the contributions of protons, neutrons, and heavy ions, are pre-requisites for human exploration. The MARIE experiment on the Mars-01 Odyssey spacecraft consists of a spectrometer to make such measurements in Mars orbit. MARIE is measuring the galactic cosmic ray energy spectra during the maximum of the 24^th solar cycle, and studying the dynamics of solar particle events and their radial dependence in orbit of Mars. The MARIE spectrometer is designed to measure the energy spectrum from 15 to 500 MeV/n, and when combined other space based instruments, such as the Advanced Composition Explorer (ACE), would provide accurate GCR spectra. Similarly, observations of solar energetic particles can be combined with observations at different points in the inner heliosphere from, for example, the Solar Heliospheric Observatory (SOHO), to gain information on the propagation and radial dependence in the Earth-Mars space. Measurements can be compared with the best available radiation environment and transport models in order to improve these models for subsequent use, and to provide key inputs for the engineering of spacecraft to better protect the human crews exploring Mars.     

A cosmochemical view of cosmic rays and solar particles

The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73.     

Fine Resolution Epithermal Neutron Detector (FREND) Onboard the ExoMars Trace Gas Orbiter

ExoMars is a two-launch mission undertaken by Roscosmos and European Space Agency. Trace Gas Orbiter, a satellite part of the 2016 launch carries the Fine Resolution Neutron Detector instrument as part of its payload. The instrument aims at mapping hydrogen content in the upper meter of Martian soil with spatial resolution between 60 and 200 km diameter spot. This resolution is achieved by a collimation module that limits the field of view of the instruments detectors. A dosimetry module that surveys the radiation environment in cruise to Mars and on orbit around it is another part of the instrument.This paper describes the mission and the instrument, its measurement principles and technical characteristics. We perform an initial assessment of our sensitivity and time required to achieve the mission goal. The Martian atmosphere is a parameter that needs to be considered in data analysis of a collimated neutron instrument. This factor is described in a section of this paper. Finally, the first data accumulated during cruise to Mars is presented.     

Experimental Investigation of InSight HP<Superscript>3</Superscript> Mole Interaction with Martian Regolith Simulant

The InSight mission launches in 2018 to characterize several geophysical quantities on Mars, including the heat flow from the planetary interior. This quantity will be calculated by utilizing measurements of the thermal conductivity and the thermal gradient down to 5 meters below the Martian surface. One of the components of InSight is the Mole, which hammers into the Martian regolith to facilitate these thermal property measurements. In this paper, we experimentally investigated the effect of the Mole’s penetrating action on regolith compaction and mechanical properties. Quasi-static and dynamic experiments were run with a 2D model of the 3D cylindrical mole. Force resistance data was captured with load cells. Deformation information was captured in images and analyzed using Digitial Image Correlation (DIC). Additionally, we used existing approximations of Martian regolith thermal conductivity to estimate the change in the surrounding granular material’s thermal conductivity due to the Mole’s penetration. We found that the Mole has the potential to cause a high degree of densification, especially if the initial granular material is relatively loose. The effect on the thermal conductivity from this densification was found to be relatively small in first-order calculations though more complete thermal models incorporating this densification should be a subject of further investigation. The results obtained provide an initial estimate of the Mole’s impact on Martian regolith thermal properties.     

Neutron Monitor Design Improvements

The original design by J. A. Simpson of the neutron monitor enabled continuous monitoring of the primary cosmic-ray flux by ground-based recordings of the nucleonic component with only a rather simple correction for atmospheric effects. Simpson (1957) extended the original pile to the 12 counter IGY neutron monitor which was deployed in a world wide network during the International Geophysical Year 1957/8. The desirability for monitors with higher counting rates became evident soon afterwards. Subsequently the NM64 super neutron monitor was designed by H. Carmichael for deployment in time for the International Quiet Sun Year 1964. Using unusually large ¹⁰BF₃ proportional counters made at Chalk River, Hatton and Carmichael (1964) studied comprehensively the experimental design of the NM64. Consequently the efficiency of neutron counters to record evaporation neutrons produced in the lead of a monitor increased from 1.9% for the IGY to 5.7% for the NM64, an increase of 3.3 times the counting rate per unit area of lead producer. During the years much attention was given to the neutron multiplicity spectrum in neutron monitors. This spectrum is related to the energy spectrum of the nucleonic component incident on the neutron monitor, but is only weakly dependent on the spectrum of galactic cosmic rays at the top of the atmosphere. Contrary to galactic cosmic rays, solar flare protons and neutrons are observed predominantly as single counts per interaction, in multiplicity 1, because of the softness of solar flare particle energy spectra. Neutron monitors have also been specially designed to record solar neutrons with increased sensitivity. Newly developed ³He counters with a largely reduced thermal neutron absorption mean free path should lead to improved efficiency in recording primary cosmic radiation. Design criteria are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

X-Rays From Mars

X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity, confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states. With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying processes in the Martian exosphere on a global scale.     

Solar Influence on Earth's Climate

An increasing number of studies indicate that variations in solar activity have had a significant influence on Earth's climate. However, the mechanisms responsible for a solar influence are still not known. One possibility is that atmospheric transparency is influenced by changing cloud properties via cosmic ray ionisation (the latter being modulated by solar activity). Support for this idea is found from satellite observations of cloud cover. Such data have revealed a striking correlation between the intensity of galactic cosmic rays (GCR) and low liquid clouds (<3.2 km). GCR are responsible for nearly all ionisation in the atmosphere below 35 km. One mechanism could involve ion-induced formation of aerosol particles (diameter range, 0.001–1.0 μm) that can act as cloud condensation nuclei (CCN). A systematic variation in the properties of CCN will affect the cloud droplet distribution and thereby influence the radiative properties of clouds. If the GCR-Cloud link is confirmed variations in galactic cosmic ray flux, caused by changes in solar activity and the space environment, could influence Earth's radiation budget. This revised version was published online in August 2006 with corrections to the Cover Date.     

Biological implications of the Viking mission to Mars

A central purpose of Viking was to search for evidence that life exists on Mars or may have existed in the past. The missions carried three biology experiments the prime purpose of which was to seek for existing microbial life. In addition the results of a number of the other experiments have biological implications: (1) The elemental analyses of the atmosphere and the regolith showed or implied that the elements generally considered essential to terrestrial biology are present. (2) But unexpectedly, no organic compounds were detected in Martian samples by an instrument that easily detected organic materials in the most barren of terrestrial soils. (3) Liquid water is believed to be an absolute requisite for life. Viking obtained direct evidence for the presence of water vapor and water ice, and it obtained strong inferential evidence for the existence of large amounts of subsurface permafrost now and in the Martain past. However it obtained no evidence for the current existence of liquid water possessing the high chemical potential required for at least terrestrial life, a result that is consistent with the known pressure-temperature relations on the planet's surface. On the other hand, the mission did obtain strong indications from both atmospheric analyses and orbital photographs that large quantities of liquid water flowed episodically on the Martian surface 0.5 to 2.5 G years ago.The three biology experiments produced clear evidence of chemical reactivity in soil samples, but it is becoming increasingly clear that the chemical reactions were nonbiological in origin. The unexpected release of oxygen by soil moistened with water vapor in the Gas Exchange experiment together with the negative findings of the organic analysis experiment lead to the conclusion that the surface contains powerful oxidants. This conclusion is consistent with models of the atmosphere. The oxidants appear also to have been responsible for the decarboxylation of the organic nutrients that were introduced in the Label Release experiment. The major results of the GEX and LR experiments have been simulated at least qualitatively on Earth. The third, Pyrolytic Release, experiment obtained evidence for organic synthesis by soil samples. Although the mechanism of the synthesis is obscure, the thermal stability of the reaction makes a biological explanation most unlikely. Furthermore, the response of soil samples in all three experiments to the addition of water is not consistent with a biological interpretation.The conditions now known to exist at and below the Martian surface are such that no known terrestrial organism could grow and function. Although the evidence does not absolutely rule out the existence of favourable oases, it renders their existence extremely unlikely. The limiting conditions for the functioning of terrestrial organisms are not the limits for conceivable life elsewhere, and accordingly one cannot exclude the possibility that indigenous life forms may currently exist somewhere on Mars or may have existed sometime in the past. Nevertheless, the available information about the present Martian environment puts severe constraints and presents formidable challenges to any putative Martian organisms. The Martian environment in the past, on the other hand, appears to have been considerably less hostile biologically, and it might possibly have permitted the origin and transient establishment of a biota.     

Mars Express and MARSIS

Space Science Reviews - The Mars Express mission to be launched in 2003 will provide high resolution measurements of the Martian atmosphere and ionosphere. The neutral density, temperature, and...     

Polar rain entry of galactic electrons into the inner heliosphere?

The understanding of the relative intensity variations in cosmic ray ions and electrons with respect to solar modulation is a grand challenge for cosmic ray modulation theory. Although effects of the heliospheric neutral sheet, gradient-curvature drifts, and merged interaction regions provide qualitative explanations for observed solar cycle variations of high energy protons and ions, these effects do not account for the anomalously high intensities of high energy galactic electrons at 22-year intervals of the solar magnetic solar cycle. From the similar modulation responses of protons and heavy ions it does not appear that cosmic ray pressure effects, dominated by protons, can account for the chargesign asymmetry of cosmic ray modulation. External factors including modulation in the heliosheath and polar linkage to the interstellar magnetic field are examined as potential causes of symmetry breaking for electron modulation with respect to the solar magnetic polarity at solar minimum.     

10Be Production in the Atmosphere by Galactic Cosmic Rays

Galactic cosmic ray nuclei and energetic protons produced in solar flares and accelerated by coronal mass ejections are the main sources of high-energy particles of extraterrestrial origin in near-Earth space and inside the Earth’s atmosphere. The intensity of galactic cosmic rays inside the heliosphere is strongly influenced by the modulation of the interstellar source particles on their way through interplanetary space. Among others, this modulation depends on the activity of the Sun, and the resulting intensity of the energetic particles in the atmosphere is an indicator of the solar activity. Therefore, rare isotopes found in historical archives and produced by spallation reactions of primary and secondary hadrons of cosmic origin in the atmosphere, so-called cosmogenic nuclides, can be used to reconstruct the solar activity in the past. The production rate of ¹⁰Be, one of the cosmogenic nuclides most adequate to study the solar activity, is presented showing its variations with geographic latitude and altitude and the dependence on different production cross-sections present in literature. In addition, estimates for altitude integrated production rates of ¹⁰Be at different locations since the early nineteen sixties are shown.     

Urey: Mars Organic and Oxidant Detector

One of the fundamental challenges facing the scientific community as we enter this new century of Mars research is to understand, in a rigorous manner, the biotic potential both past and present of this outermost terrestrial-like planet in our solar system. Urey: Mars Organic and Oxidant Detector has been selected for the Pasteur payload of the European Space Agency’s (ESA’s) ExoMars rover mission and is considered a fundamental instrument to achieve the mission’s scientific objectives. The instrument is named Urey in recognition of Harold Clayton Urey’s seminal contributions to cosmochemistry, geochemistry, and the study of the origin of life. The overall goal of Urey is to search for organic compounds directly in the regolith of Mars and to assess their origin. Urey will perform a groundbreaking investigation of the Martian environment that will involve searching for organic compounds indicative of life and prebiotic chemistry at a sensitivity many orders of magnitude greater than Viking or other in situ organic detection systems. Urey will perform the first in situ search for key classes of organic molecules using state-of-the-art analytical methods that provide part-per-trillion sensitivity. It will ascertain whether any of these molecules are abiotic or biotic in origin and will evaluate the survival potential of organic compounds in the environment using state-of-the-art chemoresistor oxidant sensors.     

Mars Global Surveyor Measurements of the Martian Solar Wind Interaction

The solar wind at Mars interacts with the extended atmosphere and small-scale crustal magnetic fields. This interaction shares elements with a variety of solar system bodies, and has direct bearing on studies of the long-term evolution of the Martian atmosphere, the structure of the upper atmosphere, and fundamental plasma processes. The magnetometer (MAG) and electron reflectometer (ER) on Mars Global Surveyor (MGS) continue to make many contributions toward understanding the plasma environment, thanks in large part to a spacecraft orbit that had low periapsis, had good coverage of the interaction region, and has been long-lived in its mapping orbit. The crustal magnetic fields discovered using MGS data perturb plasma boundaries on timescales associated with Mars' rotation and enable a complex magnetic field topology near the planet. Every portion of the plasma environment has been sampled by MGS, confirming previous measurements and making new discoveries in each region. The entire system is highly variable, and responds to changes in solar EUV flux, upstream pressure, IMF direction, and the orientation of Mars with respect to the Sun and solar wind flow. New insights from MGS should come from future analysis of new and existing data, as well as multi-spacecraft observations.     

Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations

The heating of the upper atmospheres and the formation of the ionospheres on Venus and Mars are mainly controlled by the solar X-ray and extreme ultraviolet (EUV) radiation (λ = 0.1–102.7 nm and can be characterized by the 10.7 cm solar radio flux). Previous estimations of the average Martian dayside exospheric temperature inferred from topside plasma scale heights, UV airglow and Lyman-α dayglow observations of up to ∼500 K imply a stronger dependence on solar activity than that found on Venus by the Pioneer Venus Orbiter (PVO) and Magellan spacecraft. However, this dependence appears to be inconsistent with exospheric temperatures (<250 K) inferred from aerobraking maneuvers of recent spacecraft like Mars Pathfinder, Mars Global Surveyor and Mars Odyssey during different solar activity periods and at different orbital locations of the planet. In a similar way, early Lyman-α dayglow and UV airglow observations by Venera 4, Mariner 5 and 10, and Venera 9–12 at Venus also suggested much higher exospheric temperatures of up to 1000 K as compared with the average dayside exospheric temperature of about 270 K inferred from neutral gas mass spectrometry data obtained by PVO. In order to compare Venus and Mars, we estimated the dayside exobase temperature of Venus by using electron density profiles obtained from the PVO radio science experiment during the solar cycle and found the Venusian temperature to vary between 250–300 K, being in reasonable agreement with the exospheric temperatures inferred from Magellan aerobraking data and PVO mass spectrometer measurements. The same method has been applied to Mars by studying the solar cycle variation of the ionospheric peak plasma density observed by Mars Global Surveyor during both solar minimum and maximum conditions, yielding a temperature range between 190–220 K. This result clearly indicates that the average Martian dayside temperature at the exobase does not exceed a value of about 240 K during high solar activity conditions and that the response of the upper atmosphere temperature on Mars to solar activity near the ionization maximum is essentially the same as on Venus. The reason for this discrepancy between exospheric temperature determinations from topside plasma scale heights and electron distributions near the ionospheric maximum seems to lie in the fact that thermal and photochemical equilibrium applies only at altitudes below 170 km, whereas topside scale heights are derived for much higher altitudes where they are modified by transport processes and where local thermodynamic equilibrium (LTE) conditions are violated. Moreover, from simulating the energy density distribution of photochemically produced moderately energetic H, C and O atoms, as well as CO molecules, we argue that exospheric temperatures inferred from Lyman-α dayglow and UV airglow observations result in too high values, because these particles, as well as energetic neutral atoms, transformed from solar wind protons into hydrogen atoms via charge exchange, may contribute to the observed planetary hot neutral gas coronae. Because the low exospheric temperatures inferred from neutral gas mass spectrometer and aerobraking data, as well as from CO⁺ ₂ UV doublet emissions near 180–260 nm obtained from the Mars Express SPICAM UV spectrograph suggest rather low heating efficiencies, some hitherto unidentified additional IR-cooling mechanism in the thermospheres of both Venus and Mars is likely to exist. An erratum to this article can be found at